The invention has particular application to a hydraulic down-the-hole piston hammer assembly directly acting against a drill bit which in turn is mechanically rotated and which is adapted to use the hydraulic fluid to. recover at least in part cuttings resulting from the actions.
The problem to which this invention is directed relates to the situation where the reciprocating piston hammer is driven by a fluid at pressure and the impacting faces between the hammer and the drill bit are within the fluid.
Conventionally the fluid is water.
The problem is that where the hammer is caused to strike a first end of the drill bit, upon removal of the striking face of the hammer, there will be caused, in view of the rapidity of the action, some cavitation which in turn will cause, upon collapsing of voids, significant stress forces in the localised vicinity of the impacting faces.
Such an effect has the capacity to effect significant and relatively rapid removal of parts of the material of the impacting surfaces.